1. Field of the Invention
The present invention relates to a high-pressure, fuel injection pipe having a relatively small diameter such as 15 mm or less, which is to be connected to the individual nozzle holders of a cylinder head for a fuel passage and a fuel pump.
2. Description of the Prior Art
The prior art includes a high-pressure fuel injection pipe which is doubled by fitting an inner tube of stainless steel on the inner circumference of a thick steel tube throughout the whole length, with a view to preventing cavitation erosion or corrosion (as will be shortly called the xe2x80x9cerosionxe2x80x9d) on the inner circumference, which erosion is caused by the injection pressure of the fuel which has become higher in recent years. The pipe material thus prepared is formed at its end portion with a connecting head for providing a face to be fitted on the seat of a mating partner, thus producing the injection pipe.
In the prior art described above, the erosion on the inner circumference can be prevented by the construction of the inner tube of stainless steel fitted on the inner circumference of the thick steel pipe. However, the fitted face formed on the connecting head is not sufficiently kept away from the erosion. Especially in case of using methanol as the fuel in recent years, the fitted face is chemically corroded. On the other hand, the outer circumference is plated with zinc for preventing it from being rusted. Especially the internal combustion engine using methanol has to anticipate frequent assemblies and disassemblies for its maintenance. As a result, there arises a problem that the plated metal film is peeled or that the fitted face is damaged or deformed by the more or less forcible assembly.
The present invention has been conceived in view of the above-specified problems accompanying the prior art and has an object to provide a high-pressure fuel injection pipe which is enabled to prevent any erosion effectively not only on the inner circumference but also on the fitted face, to free the fitted face from being damaged or deformed by the forced assembly, to eliminate any plating treatment of the outer circumference of the injection pipe for the rust prevention, and to free the fitted face from any chemical corrosion even if methanol is used as the fuel, by fitting an inner tube of a highly corrosion resisting and highly strong metal on the inner circumference of a thick outer metal and by making the outer tube of a metal having a corrosion resistance, a high strength and an excellent fatigue resistance such as stainless steel or titanium.
In order to achieve the above-specified object, according to the present invention, there is provided a high-pressure fuel injection pipe of doubled type, comprising: a thick outer tube; and an inner tube of made a metal and fitted in said outer tube to form a doubled pipe which is formed at its end portion with a connecting head having a face to be fitted on a seat of a mating partner, wherein the improvement resides: in that said outer tube is made of stainless steel or titanium; and in that said inner tube is formed of a thin metal having a high corrosion-resistance and a high strength and is doubled on the inner circumference of said outer tube throughout its entire length. The metal inner tube is made of stainless steel, a nickel based alloy, titanium or a titanium alloy.
According to the present invention, the thick outer tube of stainless steel or titanium and the metal inner tube of high corrosion resistance and high strength are doubly drawn by an integral reduction, with the inner tube being loosely fitted in the inner circumference of the outer tube, so that they are diametrically reduced in their entirety and are fixedly doubled. After this, this doubled pipe is formed at its end portion with a connecting head. For these treatments, the outer tube is made of a metal having a corrosion resistance, a strength and an excellent fatigue resistance such as stainless steel or titanium, and the inner tube is made of a material having a high corrosion resistance and a high strength such as: stainless steel; a nickel based alloy such as Inconel, Incoloy or Hastelloy; titanium; or a titanium alloy. As a result, the erosion can be prevented not only on the inner circumference but also on the fitted face of the connecting head at the end portion even if an alcohol fuel having a low boiling point and liable to cause an erosion is injected under a high pressure. Moreover, the fitted face can be freed from being damaged or deformed even if the injection pipe is more or less forcibly assembled. At the same time, the outer face as the injection pipe need be neither made corrosion-resistant nor plated in the least. Moreover, not only the inner circumference but also the fitted face is freed from any chemical corrosion even if the fuel used is methanol. Thus, the connection can be stabilized and ensured for a long time, and the vibration resistance can be drastically improved to provide a injection pipe for a long use.
In case the outer tube is made of titanium, the injection pipe can find a suitable use in a marine Diesel engine which is required to have a corrosion resistance to seawater and which has its natural frequency (or resonant point) increased to have a wide range of using R.P.M. and a high load fluctuation.